Enable checkstyle for REST-AT Narayana module

When working with `rts/at` module there is a trouble to pertain the checkstyle rules as the checkstyle is not enabled.

It's possible to run it manually for the module with `mvn checkstyle:checkstyle -Dcheckstyle.skip=false` but as the module itself is not fully checkstyle compliant is really hard to differentiate what are the changes which need to be fixed and what are only checkstyle issues which were part of the codebase already.

I checked the codebase and it seems the REST-AT module is pretty compliant with the WildFly checkstyle rules that we use
https://github.com/jbosstm/narayana/blob/5.11.0.Final/pom.xml#L368
and it would bee good to fix the sources and enable the checkstyle.

It will make less troubles for any contributor in future.